Communication devices such as User Equipments (UEs) are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two UEs, between a UE and a regular telephone and/or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
UEs may further be referred to as wireless terminals, mobile terminals and/or mobile stations, mobile telephones, cellular telephones, laptops, tablet computers or surf plates with wireless capability, just to mention some further examples. The UEs in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another wireless terminal or a server.
The communications network covers a geographical area which is divided into cell areas, wherein each cell area is being served by a transmission point. A cell is the geographical area where radio coverage is provided by the transmission point.
The network node may further control several transmission points, e.g. having Radio Units (RRUs). A cell can thus comprise one or more network nodes each controlling one or more transmission/reception points. A transmission point, also referred to as a transmission/reception point, is an entity that transmits and/or receives radio signals. The entity has a position in space, e.g. an antenna. A network node is an entity that controls one or more transmission points. The transmission point may e.g. be a Radio Base Station (RBS), eNB, eNodeB, NodeB, B node, Base Transceiver Station (BTS), Radio Access (RA) node. The network node may e.g. also be a base station such as a Radio Base Station (RBS), eNB, eNodeB, NodeB, B node, Base Transceiver Station (BTS), Radio Access (RA) node, depending on the technology and terminology used. The network node could however also be a distributed node comprised in a cloud and being configured to control the transmission point from a distance.
The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size.
Further, each network node may support one or several communication technologies. The network nodes communicate over the air interface operating on radio frequencies with the UEs within range of the network node. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the UE to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks. In LTE the cellular communication network is also referred to as Evolved Universal Terrestrial Radio Access Network (E-UTRAN).
An E-UTRAN cell is defined by certain signals which are broadcasted from the eNB. These signals contain information about the cell which can be used by UEs in order to connect to the network through the cell. The signals comprise reference and synchronization signals which the UE uses to find frame timing and physical cell identification as well as system information which comprises parameters relevant for the whole cell.
A UE needing to connect to the network must thus first detect a suitable cell, as defined in 3GPP TS 36.304 v11.5.0. The UE can be in either idle state, which is also referred to as IDLE or Radio Resource Control Idle (RRC_IDLE), or in connected state, which state is also referred to as CONNECTED or Radio Resource Control Connected (RRC_CONNECTED). When the UE is in RRC_IDLE, it monitors a paging channel, which paging channel is part of a Paging Control Channel (PCCH) at a logical level, a Paging Channel (PCH) on a transport channel level and a Physical Downlink Shared Channel (PDSCH) on a physical channel level. While doing so the UE typically also performs a number of radio measurements which the UE uses to evaluate the best cell, such as Reference Signal Receive Power (RSRP), Reference Symbol Received Quality (RSRQ) or Received Signal Strength Indicator (RSSI). This is performed by measuring on received reference signals and/or parts of a spectrum which comprises reference signals sent by cells. This may also be referred to as “listening” for a suitable cell.
A suitable cell is commonly a cell which has RSRQ or RSRP above a certain level. The cell with the highest RSRP or RSRQ may be referred to as the best cell or the best suitable cell. Listening for a suitable cell may comprise searching for reference signals transmitted from the network node in an Orthogonal Frequency-Division Multiplexing (OFDM) subframe. When the best suitable cell is found the UE performs random access, according to a system information for the cell. This is done in order to transmit a Radio Resource Control (RRC) connection setup request to the network node. Assuming the random access procedure succeeds and the network node receives the request, the network node will either answer with an RRC connection setup message, which acknowledges the UE's request and tells it to move into RRC_CONNECTED state, or an RRC Connection reject, which tells the UE that it may not connect to the cell. In RRC_CONNECTED state the parameters necessary for communication between the network node and the UE are known to both entities and a data transfer between the two entities is enabled.
When the UE is in RRC_CONNECTED state the UE continues to measure RSRP, as an input to connected mode mobility decisions, such as e.g. deciding when to perform a handover from one cell to another. These measurements are typically performed in the full bandwidth of the subframe, which may also be referred to as the full spectrum.
RSRP is a measurement of the signal strength of an LTE cell which helps the UE to rank the different cells according to their signal strength as input for handover and cell reselection decisions. The RSRP is an average of a power of all resource elements which carry Cell-specific Reference Signals (CRS) over the entire bandwidth. The RSRP is therefore only measured in OFDM symbols carrying CRS.
Signal-to-Noise Ratio (SNR) is a measure which compares the level of a desired signal S to the level of background noise N. SNR is defined as the ratio of signal power to the noise power, often expressed in decibels. A ratio higher than 1:1, which may also be referred to as the SNR being greater than 0 dB, indicates more signal than noise.
The signal-to-noise ratio, the bandwidth, and the channel capacity of a communication channel are connected by a ShannonHartley theorem. According to Shannon, the maximum possible bit rate C over a radio transmission link depends on the SNR as follows:C=Log2(1+S/N)
The highest possible SNR is further limited by an Error Vector Magnitude (EVM) which is a measure of how far away an actual complex transmission symbol is from its ideal location in the complex constellation. The EVM may be caused by a number of sources such as filter delay, insertion loss, phase noise in synthesizers, radio channel variations, and clipping of the transmitted signal. The highest possible SNR is given from the limitations in the transmitting radio, which limitations may be referred to as Error Vector Magnitude EVM. A majority of the EVM is however due to clipping in the radio.
When radio is operated close to its max power capability, the available peak-to-average-ratio (PAR) will be limited. When the transmit signal exceeds the available PAR the transmit signal will be clipped. The clipping is also done to facilitate higher mean output power which is advantageous on especially longer distances. This clipping introduces a significant amount of in-band noise as well as out-of-band spurious emissions resulting which results in a distortion in the transmit signal, e.g. the symbols in the constellation plane is offset by a vector corresponding to the transmit EVM. Hence, clipping of the signal may lead to increased EVM, which in turns means, for example, a lower possible maximum bit rate. When EVM increases, the UE will at some point have difficulties telling one symbol from another. This is especially the case for higher order modulations where the symbols in the constellation plane has a dense pattern.
In order to support the UE in connecting to a cell, which may also be referred to as accessing a cell, System Information Blocks (SIBs) are transmitted in a control channel, such as e.g. a Broadcast Control Channel (BCCH) logical channel in the downlink, which may be mapped to the PDSCH physical channel. In LTE a number of different SIBs are defined, which are characterized by the information they are carrying. For example, cell access related parameters, such as information about the operator of the cell, restrictions to what users may access the cell and the allocation of subframes to uplink/downlink are carried by SIB1. SIB1 further carries information about scheduling of other SIBs.
In order to reduce power consumption of the UE, Discontinous Reception (DRX) may be implemented. The basic mechanism in DRX is a configurable DRX cycle, which may also be referred to as a DRX pattern, in the UE. With a DRX cycle configured, the UE only monitors the control signaling during an onDuration interval of the DRX cycle. The onDuration interval may be one or more subframes, which may be referred to as an active subframe or active subframes. In the remaining subframes of the DRX cycle, the UE may switch off its receiver, which may also be referred to as the UE sleeping or as an offDuration interval of the DRX cycle. This allows for a significant reduction in power consumption, i.e. the longer the DRX cycle and the shorter the onDuration interval, the lower the power consumption will be. In some situations, if the UE has been scheduled and active with receiving or transmitting data in one subframe, it is likely that it will be scheduled again in the near future. Waiting until the next active subframe according to the DRX cycle may lead to additional delays in transmission. Hence, to reduce delays, the UE may remain in the active state for a certain configurable time after being scheduled, this may also be referred to as the active time or the DRX-InactivityTimer, as defined in 3GPP TS36.321 Ch3.1. The duration of the active time is set by an inactivity timer, which is the duration in downlink subframes that the UE waits before it switches off and re-enters offDuration from the last successful decoding of a Physical Downlink Control Channel (PDCCH). The UE may restart the inactivity timer after a single successful decoding of a PDCCH for a transmission. The time it takes for the UE to re-enter offDuration after the last transmission may also be referred to as the inactivity time.
CRS are UE known symbols that are inserted in a Resource Element (RE) of a subframe of an OFDM time and frequency grid and broadcasted by the network node via a RA node. Each RE has an extension in the frequency domain corresponding to an OFDM sub carrier and an extension in the time-domain corresponding to an OFDM symbol interval.
The CRSs are used by the UE for downlink channel estimation. Channel estimation is used for demodulation of downlink data both when the UE is in an RRC_CONNECTED state and is receiving user data and when the UE is in an RRC_IDLE state and is reading system information.
Due to the latter use case, the CRSs must be transmitted even from cells which do not have any UEs in RRC connected state since the network node cannot know whether a UE wants to access the network until it performs random access. Downlink CRSs are inserted within the first and third last OFDM symbol of each slot with a frequency domain spacing of six sub-carriers. A slot is a time period of the OFDM time and frequency grid, which is usually 0.5 msec long. A problem with the known technology is therefore that cells without any UEs in RRC connected state still consume power due to CRS broadcasting.
In case several antennas are used by the network node for transmitting and each antenna is representing a cell, each antenna has to transmit a unique reference signal in order for the UE to connect to that specific cell. When one antenna transmits, the other antennas have to be silent in order not to interfere with the first antennas reference signal. The CRS may be non-shifted or shifted. In the non-shifted configuration the same time and frequency resources are used for CRS transmission in all cells. Hence the CRS of the cells may interfere with each other. To reduce the interference of reference signals between the cells, the position of the CRS may however be shifted in frequency between the cells, which may be referred to as shifted CRS. The CRS may be shifted between 0-5 sub carriers, each sub carrier corresponding to a frequency shift of 15 kHz for LTE. The frequency shift may be derived from the physical Cell Identity (Cell ID) which is signaled to the UE by selection of appropriate Primary Synchronization Channel (PSCH) and Secondary Synchronization Channel (SSCH).
Although this reduces the interference of reference symbols, such as CRS symbols, between cells, it has the problem that the reference symbols of one cell will disturb PDSCH and PDCCH symbols of neighboring cells.
Hence, even though cells do not have any UEs in RRC_CONNECTED state, disturbance may impact UE DL throughput in neighboring cells. This will especially be the case when the UE is in and/or close to borders between cells.
Reducing the power of the CRS may mitigate this problem. However, in order to access a cell the UE must be able to hear the CRS of the cell, i.e. the UE must be able to recognize and receive the CRS transmitted from the cell. Therefore reducing the power of the CRS also shrinks the size of the cell, since more distant UEs no longer will hear the CRS sent by the cell. Furthermore, the quality of the channel estimates used for demodulation decreases when the Signal to Interference Ratio (SINR) on the CRS decreases. Reducing the power of the CRS therefore causes degradation of cell edge performance. This degradation is further aggravated when the load in the network increases, especially if the data is transmitted with higher power than the CRS, which is usually the case when the effect of CRS interference is to be reduced, thus, leading to a reduced performance of the wireless communication network.